Add polyfill changes for weekOfYear()

polyfill/lib/calendar.mjs

@@ -459,6 +459,14 @@ function resolveNonLunisolarMonth(calendarDate, overflow = undefined, monthsPerY
   return { ...calendarDate, month, monthCode };
 }
 
+function weekNumber(calendarHelper, desiredDay, dayOfPeriod, dayOfWeek) {
+  var periodStartDayOfWeek = (dayOfWeek - calendarHelper.getFirstDayOfWeek() - dayOfPeriod + 1) % 7;
+  if (periodStartDayOfWeek < 0) periodStartDayOfWeek += 7;
+  var weekNo = MathFloor((desiredDay + periodStartDayOfWeek - 1) / 7);
+  if (7 - periodStartDayOfWeek >= calendarHelper.getMinimalDaysInFirstWeek()) ++weekNo;
+  return weekNo;
+}
+
 // Note: other built-in calendars than iso8601 are not part of the Temporal
 // proposal for ECMA-262. An implementation of these calendars is present in
 // this polyfill in order to validate the Temporal API and to get early feedback
@@ -1702,6 +1710,12 @@ const helperGregory = ObjectAssign(
       if (era === 'bc' || era === 'b') era = 'bce';
       if (era === 'ad' || era === 'a') era = 'ce';
       return { era, eraYear };
+    },
+    getFirstDayOfWeek() {
+      return 1;
+    },
+    getMinimalDaysInFirstWeek() {
+      return 1;
     }
   }
 );
@@ -2092,7 +2106,37 @@ const nonIsoGeneralImpl = {
     return diffDays + 1;
   },
   weekOfYear(date) {
-    return impl['iso8601'].weekOfYear(date);
+    // Supports only Gregorian calendar; can be updated to add support for other calendars.
+    // Returns undefined for calendars without a well-defined week calendar system.
+    if (this.helper.id === 'gregory') {
+      const dayOfWeek = this.dayOfWeek(date);
+      const dayOfYear = this.dayOfYear(date);
+      var relDow = (dayOfWeek + 7 - this.helper.getFirstDayOfWeek()) % 7;
+      // Assuming the year length is less than 7000 days.
+      var relDowJan1 = (dayOfWeek - dayOfYear + 7001 - this.helper.getFirstDayOfWeek()) % 7;
+      var woy = MathFloor((dayOfYear - 1 + relDowJan1) / 7);
+      if (7 - relDowJan1 >= this.helper.getMinimalDaysInFirstWeek()) {
+        ++woy;
+      }
+      if (woy == 0) {
+        var prevDoy = dayOfYear + this.daysInYear(date.add({ years: -1 }));
+        woy = weekNumber(this.helper, prevDoy, dayOfYear, dayOfWeek);
+      } else {
+        var lastDoy = this.daysInYear(date);
+        if (dayOfYear >= lastDoy - 5) {
+          var lastRelDow = (relDow + lastDoy - dayOfYear) % 7;
+          if (lastRelDow < 0) {
+            lastRelDow += 7;
+          }
+          if (6 - lastRelDow >= this.helper.getMinimalDaysInFirstWeek() && dayOfYear + 7 - relDow > lastDoy) {
+            woy = 1;
+          }
+        }
+      }
+      return woy;
+    } else {
+      return undefined;
+    }
   },
   yearOfWeek(date) {
     return impl['iso8601'].yearOfWeek(date);